Spatial Patterns of Groundwater Biogeochemical Reactivity in an Intertidal Beach Aquifer

Abstract

Beach aquifers host a dynamic and reactive mixing zone between fresh and saline groundwater of contrasting origin and composition. Seawater, driven up the beachface by waves and tides, infiltrates into the aquifer and meets the seaward‐discharging fresh groundwater, creating and maintaining a reactive intertidal circulation cell. Within the cell, land‐derived nutrients delivered by fresh groundwater are transformed or attenuated. We investigated this process by collecting pore water samples from multilevel wells along a shore‐perpendicular transect on a beach near Cape Henlopen, Delaware, and analyzing solute and particulate concentrations. Pore water incubation experiments were conducted to determine rates of oxygen consumption and nitrogen gas production. A numerical model was employed to support field and laboratory interpretations. Results showed that chemically sensitive parameters such as pH and ORP diverged from salinity distribution patterns, indicating biogeochemical reactivity within the circulation cell. The highest respiration rates were found in the landward freshwater‐saltwater mixing zone, supported by high dissolved inorganic carbon. Chlorophyll a, a proxy for phytoplankton, and particulate carbon did not co‐occur with the highest respiration rates but were heterogeneously distributed in deeper and hypoxic areas of the cell. The highest rates of N2 production were also found in the mixing zone coinciding with elevated O2 consumption rates but closer to the lower discharge point. Model results were consistent with these observations, showing heightened denitrification in the mixing zone. The results of this work emphasize the relationship between the physical flow processes of the circulation cell and its biogeochemical reactivity and highlight the environmental significance of sandy beaches.

Hydrologic Shifts Create Complex Transient Distributions of Particulate Organic Carbon and Biogeochemical Responses in Beach Aquifers

“…ORP patterns, reflecting the net effects of oxidation and reduction within the beach aquifer, did not covary with salinity, consistent with previous findings at this site (Figure , right column; Kim et al, ). More reducing conditions were found toward the discharge zone and in some months, near the landward FW‐SW mixing zone (May and September 2015).…”

“…The cross‐sectional distributions of dissolved oxygen had higher saturation levels located on the upper part of the circulation cell, where the highest rates of infiltration occurred, as indicated by salinity (Figure , middle column). Seawater infiltrates the beach aquifer at fully oxygenated conditions, so the decrease in dissolved oxygen saturation along the circulating flow path indicates active consumption of oxygen (see also Kim et al, ). Overall, more dissolved oxygen was available within the intertidal circulation cell during colder months when O 2 is more soluble, biogeochemical reactions were slower, and less seawater carbon from primary production was available.…”

“…Heiss and Michael (2014) measured and modeled the dynamics of the cell over tidal, spring-neap, and seasonal cycles and showed that the most prominent changes to the cell geometry occurred over seasonal cycles. The spatial distributions of reactions within the intertidal circulation cell and their relationships to its flow field have also been documented by Kim et al (2017), with higher oxic respiration rates near the point of seawater infiltration at the beach surface and along the landward edge of the FW-SW mixing zone at depth. Because the intertidal circulation cell at Cape Shores displays migrations seasonally, and reaction rates related to the flow field have been well characterized, this field site is well suited for further research on how reaction rates in the aquifer are related to the flow and sedimentary characteristics in the aquifer over seasonal time scales.…”

Physical Controls on Biogeochemical Processes in Intertidal Zones of Beach Aquifers

“…Seasonal variability in the supply of freshwater and saltwater, and thus the supply of solutes to the reaction zone also affects the redox conditions of beach aquifers (Beck et al, ; Charbonnier et al, ; McAllister et al, ; Santos et al, ), as does the pH of the pore water (Spiteri et al, ). While the spatial and temporal patterns of biogeochemical reaction zones are becoming clearer, insight gained through field studies inherently considers the cumulative effects of multiple forcings acting on different time scales (e.g., Kim et al, ; Reckhardt et al, ). Compounded by the complexity of flow and transport in heterogeneous systems, cumulative and potentially nonlinear interactions mask the importance of the individual factors regulating nutrient cycling in these hydrologically and biogeochemically complex and dynamic coastal systems.…”

Subsurface Flow and Moisture Dynamics in Response to Swash Motions: Effects of Beach Hydraulic Conductivity and Capillarity

“…Despite the difficulty in simulation, high-frequency flow and moisture dynamics forced by wave swash are important for understanding aspects of beach aquifer systems such as subsurface geochemical conditions, particulate transport, chemical fate, surface evaporation dynamics, and aeolian sediment transport (Geng et al, 2015(Geng et al, , 2016bHuettel et al, 1996;Kim et al, 2017;Malott et al, 2017;Molnar et al, 2015;Nickling & Davidson-Arnott, 1990;Wu et al, 2017). Wave swash strongly controls the flux of water into beach aquifers, as well as flow paths, residence times, and mixing of various chemical species beneath the swash zone (Geng et al, 2014;Malott et al, 2016;Robinson et al, 2014;Xin et al, 2010).…”